Single-phase and two-phase motors are widely used in a broad range of applications, from automotive applications to white appliances or industrial applications, as well as consumer applications like CPU cooling, and power supply cooling. Despite the fact that such motors are widely applied, and many people are able to apply existing designs, only a very small group of people has sufficient knowledge and experience to actually design a motor driver for single-phase motors.
The startup of a motor comprises applying a current in the stator coil in a specific direction for a single-phase motor, or to draw current through the applicable coil of a 2-phase motor. The direction of the current (or which coil for 2-phase motors) is depending on the position of the rotor versus the stator, and more specific the position of the individual poles of the permanent magnets versus the stator, in order to guarantee the start up in the correct direction. In single-phase motors, the coil is wound such that the stator shoes for a given current direction cause magnet fields in alternating directions-field lines from stator shoe towards the rotor, or vice versa. In some two-phase motors, the 2 coils are wound on alternating stator poles.
Typically, single-phase and two-phase motors have rotors with two pole pairs, or four poles. The magnetic poles on the rotor are per default also alternating North and South poles. Consequently, the two North poles and South poles face stator shoes with coils wound in the same direction. In general, and especially after power-on-reset, the initial position of the rotor is unknown.
In the prior art the typical method to detect which pole is facing which coil winding direction, is based on a hall sensor which is applied in the vicinity of the rotor, and which measures the local magnetic field. This informs the fan-driver in which direction the current must be applied through the coil to ensure correct start up direction. If the motor is started in the “incorrect” direction, the rotor may become stuck in a zero-torque position before having gained sufficient inertia. This is a first fundamental difference between for example three-phase BLDC motors on the one hand, and single-phase and two-phase motors on the other hand, because zero torque positions exist for the single-phase and two-phase motors (as illustrated in FIG. 13), but not for three-phase motors, and the rotor has the tendency to stop at such position because it is the position of lowest reluctance as well. Hence, techniques that are known for three-phase motors do not by definition also work for single-phase motors.
Typically, 1-coil and 2-coil motors have four stator shoes, as illustrated in the exemplary motor shown in FIG. 14. Each stator shoe causes such a zero torque position, so there are four zero-torque positions per motor. In order to ensure startup, an asymmetrical air gap (distance “gap1” being different from distance “gap2”) is typically provided by design in single coil fans, to ensure the fan stops next to the zero torque position, more specifically on that side of the zero torque position which is the most favorable start up direction, i.e. the position further away from the “next” zero torque position, so that the motor will have gained sufficient inertia for passing this zero torque position, provided that the correct start-up sequence is applied. In case the incorrect current direction is applied to the stator coil, the rotor will be pulled into the zero torque position, which may lead to stall (locked rotor), or a start-up in the incorrect direction.
Rotating in the correct direction is important, because single-coil motors are often used for driving a fan, and the blades of fans are typically designed to rotate in one specific direction. Hence, if the motor is started-up incorrectly, and rotates in the incorrect direction, the wind is flowing in the wrong direction, and the fan will not provide the envisioned cooling, which is highly undesirable.
Another problem with single-coil or two-coil motors is that there is no simple way, other than by using the position sensor, e.g. Hall sensor, to detect in which direction the rotor is running. Hence, simply omitting the position sensor, and trying to start-up in a random way, and if not correct, trying again, is not an option.
Therefore, it is important to know the position of the rotor in order to correctly start a single-phase or two-phase motor.
As far as is known to the inventors, most or all existing single-coil or dual-coil motors without parallel wound coils use position sensors, such as Hall sensors, to determine the initial position, simply because it is the only reliable technique known today, despite the fact that such a sensor increases the cost of the motor driver, e.g. fan-driver electronics.
There is always room for improvements or alternatives.